1. Field of the Invention
Recently, sintered ferrous materials have found wide application as structural components in machines. However, such materials are rarely used as the main structural components of machines. The failure to use sintered materials as main structural components stems from the porous character of such components which are inferior in mechanical properties to casting or forged materials having the same composition. To promote the use of sintered materials in such components, various efforts have been made to increase the density of the sintered material up to a value close to the theoretical density. The present invention employs a specially selected iron-titanium alloy in a liquid phase sintering method which produces excellent machine components.
2. Description of the Prior Art
In one attempt to decrease the porosity of sintered material and to increase its density, the pressure employed in compressing the powders has been increased. In one instance, the density of a green compact obtained under a compacting pressure of 4 ton/cm.sup.2 was about 78 percent of the solid iron metal, and by increasing the pressure to 8 ton/cm.sup.2, the density increased to about 90 percent of the solid iron metal. Though a higher density was achieved by increasing the compacting pressure, the wear of the metal dies was accelerated and this, in turn, had an adverse effect on the economics of the operation. Accordingly, the pressure employed in compacting operations of powders is, for practical purposes, limited to between 4 to 6 ton/cm.sup.2, at the most, and in such instances, the density of the green compacts formed is usually on the order of 6.0- 6.6 g/cm.sup.3. When such compacts are sintered, the progress of sintering is very slow, because of the solid phase sintering and, as a result, little shrinkage is obtained, and it is very difficult to achieve a denser sintered material.
In other efforts to obtain a more dense sintered product the following processes have been employed: (1) a method employing repeated compressing and sintering steps, (2) a liquid phase sintering method, and (3) an infiltration method, however, each of these processes has disadvantages which render its use impractical.
1. In the method employing repeated compressing and sintering steps, the considerable number of separate operations required to obtain a suitably high density material has made this process very expensive.
2. In the liquid phase sintering method elements, such as phosphorus, boron, copper and others, have been included with the iron to form a liquid phase at the sintering temperature and, as the amount of the element is increased, the degree of sintering is improved and the final product becomes more dense. However, when more than 0.8% by weight of phosphorus and/or more than 0.2% by weight of boron are used, the product obtained is very brittle because hard eutectic phases remain at the grain boundaries. Furtherm if copper is added, the ductility of the sintered product is adversely affected due to the intrinsic properties of the copper. As a result, in the past where liquid phase sintering methods have been used, the sintered products achieved have been very brittle.
3. In the infiltration method, the pores of the sintered solid are filled with copper in the fused state by a capillary phenomenon and, as a result, the density of the sintered material is increased. However, as indicated above, the difficulty of sintered materials containing an addition of copper is adversely affected.
The Canadian Pat. No. 599,259 teaches a method of making porous metal piston rings of nickel-titanium alloy. These components of machines are relatively porous and not suitable for the purposes of the present invention which yields a structural component of higher density and strength than that of Canadian Pat. No. 599,259 which is a forged product formed of a hard nickel-titanium-iron ternary alloy wherein there is some fusion of nickel-titanium alloy at higher sintering temperatures. The present invention permits formation of a machine component of high density, and low porosity without the necessity of a forging step by selection of critical amounts of titanium-iron alloy components and carrying out sintering completely in the liquid phase.